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Part 6 of 7 · Cash drawer reconciler series ~5 min read

What the cash drawer reconciler costs

A shop with two tills trading six days a week makes about fifty closes a month; a small chain with five sites makes a few hundred. Nothing here is always-on, and the messaging line is unusually small because the whole design is about not sending messages. Here is where each cent goes.

Key takeaways

  • About $3 a month at 180 closes. Roughly $6 at 600 closes.
  • One Bedrock read per close is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • The messaging line is tiny on purpose: a good month sends almost no mail at all.
  • The duplicate test runs before the read, so resends are free.
  • The three real risks: a retry loop, storage nobody expires, and a bigger model than the job needs.

The bill at three volumes

These are US East prices at the time of writing, at three volumes that bracket most small businesses. Find the bar closest to your own and read across.

Monthly cost of the cash drawer reconciler at three volumesA stacked bar chart with three bars, one per volume tier: 60 closes totalling about $2, 180 closes totalling about $3, and 600 closes totalling about $6. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per close, in teal. Then SES for the messages, in pink. Then S3 and DynamoDB storage in green. Then a fixed orange band for Secrets Manager and AWS Budgets, which is eighty-six cents at every volume. Then a grey band for Lambda, SQS and CloudWatch. A note says the read is the only bar that grows with the business and the orange band never moves.$0$2$4$6$8~$1.7660 closes~$2.81180 closes~$6.49600 closesBedrock — one read per closeSES — asks, results, receiptsTextract — count sheets and printoutsS3 + DynamoDBFixed — Secrets Manager, BudgetsLambda, SQS, CloudWatchThe read is the only bar that grows with the business. The orange fixed band never moves.
Fig 1. The monthly bill at three volumes. The teal band — one model read per close — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 180 closesHow it scales
Bedrock read$0.61Linear. One call per close, roughly 1,800 in and 200 out tokens.
Textract$0.68Linear at $0.0038 per close.
SES$0.10Linear. About 0.4 messages per close.
DynamoDB + S3$0.31Storage grows with what you retain, not with throughput.
Lambda + SQS$0.12Linear, and effectively free at this scale.
CloudWatch$0.16Flat, if you set retention. Unbounded if you do not.
Secrets Manager$0.40Flat. One secret, $0.40 a month.
AWS Budgets$0.46Flat. Two actions, so you find out before the bill does.

The Textract line is the biggest per-close cost here, because most closes involve two photographs rather than one — the count sheet and, where the point of sale only prints, the till printout. A POS that emails its export halves it.

The three ways this bill surprises you

Every one of these has happened to somebody, and all three are cheap to prevent.

  • A retry loop on an unreadable photo. A dark or angled count sheet makes Textract return nothing useful, the function throws, and the queue redelivers. Without a dead-letter queue that is one bad photo costing more than a month of good ones. A maximum receive count of three fixes it.
  • Re-reading on every recount. A supervisor recounting and rephotographing three times pays for three full reads unless the cache is keyed on the image digest rather than the shift id.
  • Keeping every photo forever. Count sheets are small but a hundred and twelve a month for seven years is not. An S3 lifecycle rule tiering at ninety days and expiring at your actual record-keeping horizon keeps the storage line flat.

What it costs when nothing happens

This matters more than the headline number for a seasonal business. In a month with nothing to process the bill is the fixed band: Secrets Manager at forty cents, AWS Budgets at forty-six, and a few cents of storage. Call it a dollar. There is no instance to stop and nothing to remember to turn off.

The monthly bill at four volumes plus one failure modeA horizontal row of five boxes. Quiet month, about one dollar. 60 closes, about $2. 180 closes, about $3. 600 closes, about $6. And one bad retry loop, about two hundred dollars. A note says four of these are the design working and the fifth is a missing dead-letter queue.THE BILL, AT A GLANCEQuiet month~$160 closes~$2180 closes~$3600 closes~$6One bad loop~$200Four of these are the design working. The fifth is a missing dead-letter queue.
Fig 2. The bill at a glance, including the one that is not a volume at all. A retry loop with no dead-letter queue costs more than every legitimate use of the system put together.
  • Management
  • Analytics
  • Front-end & mobile

Set these on day one

  • A dead-letter queue on every SQS queue, with a maximum receive count of three.
  • Thirty-day retention on every CloudWatch log group. There is no default that is safe.
  • An S3 lifecycle rule on the object prefix, tiering at 90 days and expiring at your actual record-keeping horizon.
  • Two AWS Budgets actions — one that emails at half your expected spend, one at double it. The second is how you find out about a loop in an hour instead of a month.
  • Provisioned concurrency: none. Nothing here is latency-sensitive enough to justify paying for a warm function.

Next: the same system drawn for engineers — service names, resource identifiers, IAM scopes, table schemas and the model id.

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